A new alternative system for the oxidation of

secondaryalcoholstoketoneswithDMSO/N2H4‚H2O/I2/H2O/

CH3CN in hydrated media has been developed. The system

also selectively oxidizes the secondary alcoholic groups to

the corresponding ketones in the presence of primary

alcoholic groups present within the same molecule in moderate

to very good yields at reflux temperature

GOGOI , P

KONWAR , D

SARMAH , G K

Gogoi

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DMSO/N2H4×H2O/I2/H2O/CH3CN: A New System for Selective Oxidation of Alcohols in Hydrated Media.

IR@NEIST - North East Institute of Science and Technology (CSIR)

DMSO/N2H4âH2O/I2/H2O/CH3CN: A NewSystem for Selective Oxidation of Alcoholsin Hydrated MediaPranjal Gogoi, Gautam Kumar Sarmah, andDilip Konwar*Organic Chemistry Division, Regional Research Laboratory,Jorhat 785006, Assam, Indiadkonwar @yahoo.co.ukReceived March 27, 2004Abstract: A new alternative system for the oxidation ofsecondary alcohols to ketones with DMSO/N2H4âH2O/I2/H2O/CH3CN in hydrated media has been developed. The systemalso selectively oxidizes the secondary alcoholic groups tothe corresponding ketones in the presence of primaryalcoholic groups present within the same molecule in moder-ate to very good yields at reflux temperature.Oxidation of alcohols to carbonyl compounds is one ofthe most important transformations in organic synthe-ses.1 The selective oxidation of alcohols is still a chal-lenging task to organic chemists particularly when bothsecondary and primary groups within the same moleculeare present. Many selective reagents for the oxidation ofsecondary alcohols to ketones have been reported, includ-ing halogen-based oxidants,2a e.g., N-chloroacetamide,N-chloro/bromosuccinaamide, Cl2/pyridine, Br2/HMPT/NaHCO3, (Bu3Sn)2O/Br2, NaOCl/CH3COOH, NaBrO3/NaHSO3, and CeSO4/NaBrO3. Other important selectiveoxidizing agents viz. Mo/Zr/W/Ru compounds, peroxides,dioxiranes, and enzymatic methods are also efficientreagents for the selective oxidation of secondary alcohols.2bOxidation by DMSO and related methods3 is widely usedin organic syntheses, although few examples of primary-secondary selectivity have been reported.4 Aliphaticprimary and secondary alcohols were oxidized in thepresence of allylic or benzylic alcohols using the DMSO/(CF3CO)2/Et3N5 system. The competitive oxidation ofalcohols with Me2SO/(COCl)2/Et3N at -60 °C has dem-onstrated that alcohols bearing electron-deficient groupsare generally less reactive and secondary alcohols wereoxidized at faster rates than primary.6 However, themethods of oxidation by DMSO have disadvantagesbecause of their involvement under anhydrous3 and low-temperature conditions,6 use of moisture-sensitive3c-f,5,6and toxic reagents3e and bases,5,6 and in some cases,occurrence of Pummerer rearrangements3a (up to 50%).Therefore, development of a better selective reagent isdesirable.In our previous communications, we reported deoxy-genation of nitrones and azoxy compounds to the corre-sponding imines and azo compounds catalyzed by HI7(generated from AlCl3â6H2O/NaI/H2O/CH3CN). The samesystem was used in the dehydration of oximes and amidesto nitriles and the Beckmann rearrangements and theBischler-Napieralski reactions8 in hydrated media. Wenow report here the selective oxidation of secondaryalcohols to ketones using DMSO catalyzed by HI (gener-ated insitu from N2H4âH2O and I2)9 in hydrated mediaat reflux temperature (Scheme 1).A 1 M quantity of N2H4âH2O and a 2 M quantity of I2were stirred in aqueous acetonitrile (5:1) for a period of15 min at room temperature under a nitrogen atmo-sphere followed by addition of secondary alcohols, andthe reaction mixture was stirred for 5 min. To thismixture, was added a 5 M quantity of DMSO and themixture refluxed at 80 °C, which produced the corre-sponding ketones in 46-95% yields (Table 1).The aliphatic secondary alcohols (Table 1, entries 1a-1c) reacted faster than the phenyl-substituted aromaticsecondary alcohols (Table 1, entries 1d-1f). The rate aswell as the yield of the reactions is dependent on thesubstituents present in the substrates. Electron-donatinggroups such as (CH3)2CHCH2 or OH (Table 1, entries 1e-1g) on the aromatic ring accelerated the rate of thereaction. But aromatic primary alcohols (Table 2, entries1j and 1k) formed the corresponding aldehydes (only 3.7and 7%, respectively) after heating for 30 h at 80 °C. Thereagent selectively oxidized only the secondary alcoholgroups to the corresponding ketones when both theprimary and the secondary groups were present withinthe same molecules (Table 2, entries 1l-1o). The reagentdid not reduce alcohols to the corresponding saturatedcompounds10 or form any Pummerer rearrangement3atype of products. Moreover, oxidations of the secondaryalcohols 1a-i with DMSO catalyzed by HI (57 wt %)under the same reaction conditions produced 30-45%yields of ketones. Acetonitrile was the best solvent for(1) (a) Larock, R. C. Comprehensive Organic Transformation, Aguide to functional group preparation; VCH Publications, Inc.: NewYork, 1989; pp 604-614. (b) Ley, S. V.; Norman, J.; Griffith, W. P.;Marsden, S. P. Synthesis 1994, 7, 639-666 (c) Kim, S. S.; Jung, H. C.Synthesis 2003, 2135-2137.(2) (a) Stevens, R. V.; Chapman, K. T.; Stubbs, C. A.; Tam, W. W.;Albizati, K. F. Tetrahedron Lett. 1982, 23, 4647-4650 (b) Arterburn,J. B. Tetrahedron 2001, 57, 9765-9787 and references therein.(3) (a) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651-1660. (b)Tidwell, T. T. Synthesis 1990, 857-870. (c) Epstein, W. W.; Sweat, F.W. Chem. Rev. 1967, 67, 7-260. (d) Albright, J. D. J. Org. Chem. 1974,39, 1977-1979. (e) Albright, J. D.; Goldman, L. J. Am. Chem. Soc.1965, 87, 4214-4216. (f) Fenselau, A. H.; Moffatt, J. G. J. Am. Chem.Soc. 1966, 88, 1762-1765.(4) (a) Godfrey, J. D., Jr.; Gordon, E. M.; Langen, D. J. V. Tetrahe-dron Lett. 1987, 28, 1603 (b) Tidwell, T. T. Org. React. 1998, 39, 297-572. (c) Pfitzner, K. E.; Moffat, J. G. J. Am. Chem. Soc. 1963, 85, 3027.(d) Kornblum, N.; Jones, W. J.; Anderson G. J. J. Am. Chem. Soc. 1959,81, 4113-4114.(5) Omura, K.; Sharma, A. K.; Swern, D. J. Org. Chem. 1976, 41,957-962.(6) Marx, M.; Tidwell, T. T. J. Org. Chem. 1984, 49, 788-793.(7) Boruah, M.; Konwar, D. Synlett 2001, 6, 795-796.(8) Boruah, M.; Konwar, D. J. Org. Chem. 2002, 67, 7138-7139.(9) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements;Pergamon Press: New York, 1984; p 948.(10) Parham, W. E.; Sayed, Y. A. Synthesis 1976, 116.SCHEME 110.1021/jo0494984 CCC: $27.50 © 2004 American Chemical SocietyJ. Org. Chem. 2004, 69, 5153-5154 5153Published on Web 06/30/2004both yields and rates of the reaction, in contrast to CHCl3and CH2Cl2.While the exact mechanism of selective oxidation is notclear at present and we propose that hydrazine reducesI2 to HI,9 HI reacts with the hydroxyl group of secondaryalcohols to generate carbocations to produce ketones inthe presence of DMSO. The secondary carbocation ismore stable than primary carbocation in the transitionstate,11 which may facilitate the faster oxidation ofsecondary alcohol to ketones under the reaction condi-tions. It has been found that the system oxidized benzylalcohol to benzaldehyde (3.7%) [2,4-dinitrophenyl hydra-zone derivative, mp 236 °C (lit.12 mp 237 °C)] whenrefluxed for 30 h. DMSO is the oxidizing agent, whichwas confirmed by carrying out the oxidation under anitrogen atmosphere, and it was found that air11 did notaffect the acid-catalyzed oxidation. Also, the dimethylsulfide formed in the reaction mixture reacted withmercuric chloride to give the mercuric chloride derivativ,emp.147-49 °C (lit.1313-14 mp150-51 °C). On the basis ofthese results, a plausible mechanism is proposed (Scheme2).In conclusion, a new, selective, and efficient alternatemethod has been developed for the oxidation of secondaryalcohols to ketones in moderate to very good yields inhydrated media. It does not involve moisture-sensitiveand toxic reagents and bases in the reaction mixture. Inaddition, it can selectively oxidize only the secondaryalcoholic groups in the presence of primary alcoholspresent within the same molecule and it does not reducethe alcohols to the corresponding saturated compounds.Experimental Section:General Procedure for the Oxidation of SecondaryAlcohols. Hydrazine monohydrate (51 mg, 1 mmol) and iodine(506 mg, 2 mmol) were stirred in aqueous acetonitrile (10 mL)[CH3CN/H2O 5:1] for 15 min under a nitrogen atmosphere, andsecondary alcohol (1 mmol) was added to it. The reaction mixturewas stirred for another 5 min at room temperature. DMSO (390mg, 5 mmol) was added to the reaction mixture and refluxed at80 °C under nitrogen. The solvent was distilled off to half of itsvolume under reduced pressure, poured into water (100 mL),and extracted with diethyl ether (2  25 mL). The organic layerwas washed with 5% sodium thiosulfate solution (20 mL) andthen with water (2  50 mL) and dried over Na2SO4. The residuewas purified by preparative TLC and gave the analytically pureketones. All of the products were commercially available andidentified by comparison of the isolated products with authenticsamples.Acknowledgment. We acknowledge Dr. P. G. Rao,Director, and Dr. N. Borthakur, Senior Scientist, RRL,Jorhat-6, Assam, India, for their help. P.G. thanksCSIR, New Delhi, for a JRF fellowship.Supporting Information Available: Characterizationdata including 1H and 13C NMR and FTIR of all compounds.This material is available free of charge via the Internet athttp://pubs.acs.org.JO0494984(11) March, J. Advanced Organic Chemistry, Reactions, Mechanismsand Structure, 4th ed.; John Wiley & Sons: New York, 1999; p 166.(12) Buckingham, J.; Macdonald, F. Dictionary of Organic Com-pounds, 6th ed.; Chapman and Hall: London, 1996; Vol. 1, p 558.(13) Faragher, W. F.; Morrell, J. C.; Comay, S. J. Am. Chem. Soc.1929, 51, 2781.(14) (a) Ueno, Y.; Okawara, M. Tetrahedron Lett. 1976, 50, 4597-4600. (b) Dictionary of Organic Compounds, 6th ed.; Chapman andHall: London, 1996.TABLE 1. Oxidation of Secondary Alcoholsa Isolated yield. b Reflux temperature (80 °C). c The structuresof the compounds were determined by using 1 H NMR, FTIR, and13C NMR analyses and comparison with authentic samples.14TABLE 2. Selective Oxidation of Secondary Alcoholsa Isolated yield. b Reflux temperature (80 °C). c The structuresof the compounds were determined by using 1 H NMR, FTIR, and13C NMR analyses and comparison with authentic samples.14SCHEME 25154 J. Org. Chem., Vol. 69, No. 15, 2004

DMSO/N2H4.H2O/I2/H2O/CH3CN : A new system for selective oxidation of alcohols in hydrated media

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DMSO/N2H4.H2O/I2/H2O/CH3CN : A new system for selective oxidation of alcohols in hydrated media

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